1. Field of the Invention
The present invention relates to thermoelectric cooling devices. More particularly, but not by way of limitation, the present invention relates to a power supply and control circuit incorporating an improved design for supplying power to thermoelectric cooling devices and improved temperature control of the thermoelectric cooling devices.
2. History of the Prior Art
The development of thermoelectric cooling (TEC) devices has revolutionized the cooling industry. Conventional cooling has typically required the expansion and compression of gases, such as Chloroflorocarbons (CFC's) and Hydrochloroflorocarbons (HCFC's) to effect the absorption of heat for lowering the temperature of a fluid flowing in association therewith. Unfortunately, these substances are damaging to the earth's ozone layer and are being phased out in future years. The development of solid state cooling systems in the form of TEC devices, has permitted small commercial packages to be developed that are capable of precise temperature control in a variety of applications where environmental concern, size, weight, performance, and noise are at issue.
The most typical TEC device incorporates a thermoelectric module/component that utilizes electrical current to absorb heat from one side of the module and dissipate the heat on the opposite side. If the current direction is reversed, so is the heat pumping. Generally, cold sides and hot sides are developed necessitating an effective means of removing or adding heat from or to a solid, liquid or a gas (typically air).
An example of such an application of a TEC device is seen in U.S. Pat. No. 5,097,829 (the '829 patent) for a temperature controlled cooling system. In this patent, the advantages of medically therapeutic cooling of a wound site on a body are discussed. Initial use of cooling therapy was mainly found in the field of orthopedics. It is now found that post surgical cooling is highly beneficial in the reduction of trauma to the patient. It also increases the rate of healing and reduces the length of a hospital stay. In addition, cooling therapy is also being used in home health care for chronic pain control and to increase joint flexibility and facilitate the rate of healing.
Numerous non TEC prior art devices have been proposed for reducing the temperature of a body part in order to achieve the beneficial results obtained thereby. For example, ice packs have long been used to reduce swelling and achieve some of these benefits. In addition, cold packs containing two chemicals, which when mixed together absorb heat (endothermic reactions ), have also been used. Alternately, and more expensively, cooling pads are used through the application of a compressor, refrigerant condensing and evaporator coils. Such devices are very inconvenient and contain many inherent disadvantages.
A problem associated with the applications of very cold surfaces, such as that of an ice pack, directly to a body part is its effect on the skin. The temperature of the ice pack is very cold and can only be left against the skin for a short period of time. Generally, leaving the ice pack against the skin longer than thirty minutes can result in damage to the skin. It is much more desirable to be able to apply a temperature in a range between fifty and fifty-five degrees, which is relatively comfortable to the skin, and maintain that temperature for a substantial number of days. This prolonged application insures that the body part is cooled to the inner depth of the bone or tissue of the traumatized area. With an ice pack, cooling only takes place in the subdural area. In a more precisely controlled temperature application, cooling can take place at a deep penetration for an extended period. Thus, it is highly desirable to be able to maintain precise control of the temperature which is actually contacting the tissue of a wound site and then sustain that temperature for a substantial period of time. In this manner, the advantages obtained from the use of cold therapy in a medical application can be vastly increased. This can be done with TEC devices as shown in the '829 patent. It would be advantageous to improve the temperature control of thermoelectric cooling devices while reducing the number of parts and reducing the cost of providing improved temperature control.
One of the most ubiquitous problems in the area of industrial control is that of temperature regulation. Variations in ambient temperature, process loading, and power input must all be accounted for in such a manner that the controlled system will reach an equilibrium point in a reasonable time, and oscillations in temperature will be minimized. There are two generally used control methods or systems to accomplish this end: Thermostatic Control and Proportional-Integral-Derivative (PID) control.
The simplest method or system is thermostatic control which is exemplified by a typical home thermostat. In this system, full power is applied to the thermal generator until the desired set point is reached, whereupon the power is removed and the system "coasts" back below the set point. This system is characterized by temperature fluctuations above and below the set point but is extremely economical to implement.
The second method or system is PID and is the most common form of industrial temperature control. In this method, the output of the thermal generator is regulated in a linear fashion, such that just enough power is supplied to the load to make up for losses in the system. The amount of power applied to the load is computed from three factors, hence the term PID. The first factor is the proportional term, which is a measure of the error at any particular instant. This temperature error is multiplied by the proportional gain factor and applied in a direction such as to reduce the total error. The second factor is the integral term which is simply the time integral of the temperature error. The integral term is used to "help" the proportional term in driving the temperature error to zero. The third factor is the derivative term and is calculated as the rate of change of the temperature and is used as a "brake" on the other two terms when the temperature i s changing rapidly. These three terms are added algebraically to give the final power input value to the thermal generator. This system is capable of giving excellent temperature control where the heat load is essentially constant but requires that the temperature be known to a high degree of resolution in order to make use of the various gain terms. This system cannot regulate temperature to the degree of resolution needed for many thermoelectric systems.
The present invention provides an improvement over the prior art by providing a smooth DC voltage for the thermoelectric cooling devices and eliminates the use of bulky inductors or transformers, does not operate at high frequencies typical of switching power supplies and maintains a low part count. In addition, the improved temperature control algorithm provides tighter control as compared to a PID controller.